/*
 * Copyright 2015-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.dcp.jeedcp.common.persistence.dialect.db;


import com.dcp.jeedcp.common.persistence.dialect.Dialect;

/**
 * DB2的分页数据库方言实现
 *
 * @author poplar.yfyang
 * @version 1.0 2010-10-10 下午12:31
 * @since JDK 1.5
 */
public class DB2Dialect implements Dialect {
	@Override
	public boolean supportsLimit() {
		return true;
	}

	private static String getRowNumber(String sql) {
		StringBuilder rownumber = new StringBuilder(50).append("rownumber() over(");

		int orderByIndex = sql.toLowerCase().indexOf("order by");

		if (orderByIndex > 0 && !hasDistinct(sql)) {
			rownumber.append(sql.substring(orderByIndex));
		}

		rownumber.append(") as rownumber_,");

		return rownumber.toString();
	}

	private static boolean hasDistinct(String sql) {
		return sql.toLowerCase().contains("select distinct");
	}

	@Override
	public String getLimitString(String sql, int offset, int limit) {
		return getLimitString(sql, offset, Integer.toString(offset), Integer.toString(limit));
	}

	/**
	 * 将sql变成分页sql语句,提供将offset及limit使用占位符号(placeholder)替换.
	 * <pre>
	 * 如mysql
	 * dialect.getLimitString("select * from user", 12, ":offset",0,":limit") 将返回
	 * select * from user limit :offset,:limit
	 * </pre>
	 *
	 * @param sql               实际SQL语句
	 * @param offset            分页开始纪录条数
	 * @param offsetPlaceholder 分页开始纪录条数－占位符号
	 * @param limitPlaceholder  分页纪录条数占位符号
	 * @return 包含占位符的分页sql
	 */
	public String getLimitString(String sql, int offset, String offsetPlaceholder, String limitPlaceholder) {
		int startOfSelect = sql.toLowerCase().indexOf("select");

		StringBuilder pagingSelect = new StringBuilder(sql.length() + 100).append(sql.substring(0, startOfSelect)) //add the comment
				.append("select * from ( select ") //nest the main query in an outer select
				.append(getRowNumber(sql)); //add the rownnumber bit into the outer query select list

		if (hasDistinct(sql)) {
			pagingSelect.append(" row_.* from ( ") //add another (inner) nested select
					.append(sql.substring(startOfSelect)) //add the main query
					.append(" ) as row_"); //close off the inner nested select
		} else {
			pagingSelect.append(sql.substring(startOfSelect + 6)); //add the main query
		}

		pagingSelect.append(" ) as temp_ where rownumber_ ");

		//add the restriction to the outer select
		if (offset > 0) {
			//			int end = offset + limit;
			String endString = offsetPlaceholder + "+" + limitPlaceholder;
			pagingSelect.append("between ").append(offsetPlaceholder).append("+1 and ").append(endString);
		} else {
			pagingSelect.append("<= ").append(limitPlaceholder);
		}

		return pagingSelect.toString();
	}
}
